This invention relates to liquid level gauges, and more particularly to a liquid level gauge having a gauge plate that automatically rotates toward gravity to thereby increase accuracy of liquid level determination in high pressure containers holding liquefied gas, such as liquefied petroleum gas (LPG), propane, or other fuels that exit the tank in a gaseous state, as well as fuel tanks or other containers for holding a liquid that exits the tank in a liquid and/or gaseous state.
It is often necessary to determine the fill level of transportable high pressure storage tanks associated with transport trucks (commonly referred to as bobtails) and large horizontally mounted stationary tanks both during filling and at any time thereafter, such as when the contents of the storage tank have been transferred to one or more smaller tanks. Such high-pressure tanks often contain liquefied petroleum gas (LP-Gas) or propane, butane, natural gas, and so on, in both a liquified and gaseous states. Although some mixing of the gas and liquid may occur during transportation, normally the liquid and gaseous states are separated. Referring to FIG. 1, a common practice for determining the fill level of such storage tanks is to mount a roto-gauge 1 to a storage tank 2, typically at the rear center of the storage tank 2 where it can be manipulated and observed by an operator. As shown, the roto-gauge 1 typically includes a mounting head 3 with external threads 4 that sealably engage inner threads 5 associated with a wall 6 of the storage tank 2. A pipe or tube 7 extends through the mounting head 3 and includes a passageway 8 that extends from an inner tip 9 of the tube 7 located inside the tank to a valve 10 located outside the tank. The valve 10 is of typical construction and can be opened with a wrench or the like for allowing fluid in the liquid and/or gaseous state to flow through the passageway and exit into atmosphere. A handle 11 is connected to the tube 7 for rotating the tube about a central axis with respect to the mounting head 3. As the tube is rotated, the inner tip 9 of the tube will be moved between the gas 14A and liquid 14B (FIG. 2A) in the tank. When the inner tip 9 is in the gas 14A, the operator will be able to hear a hissing sound as gas under high pressure escapes into atmosphere through the passageway 8 As the inner tip 9 of the tube is rotated so as to be immersed in the liquid, the hissing sound will audibly change and the operator should be able to see liquid fuel escape into atmosphere under pressure. When this happens, a pointer 12 formed on the handle 11 can be observed with respect to a gauge plate 13, which typically has numerals and tick marks indicative of the fill level or volume of liquid currently in the tank. The gauge plate 13 is typically sandwiched between the mounting head 3 and a nut 14 secured against the gauge plate.
Although such devices have been used for many years, dispersing pressurized flammable vapor and liquid into the atmosphere for the purpose of ascertaining liquid level in the storage tank can impact the environment and create potential safety issues for inexperienced or distracted operators. Other drawbacks to such a system are shown in FIGS. 2A and 2B for example. During installation of the roto-gauge 1, the gauge plate 13 may be incorrectly mounted at an angle A with respect to the direction of gravity, such that the numerals on the gauge plate do not correctly match up with the actual level of liquid within the holding tank 2. As shown in FIG. 2A, an angle “A” between the 0% reading and the actual needle position (aligned with gravity or the vertical direction) indicating an empty tank illustrates the potential inaccuracy in measurement as all the gauge numbers may be skewed by the angle “A” and fail to match up when the inner tip 9 of the tube 7 is rotated between the liquid/gas interfaces on the right and left sides of the gauge plate 13. As shown in FIG. 2A, the liquid level reading is at approximately 60% when the inner tip 9 is at the liquid/gas interface on the right side of the gauge plate 13, as represented by the handle 11 and pointer 12 in solid line. However, when the handle 11 is rotated to move the tip 9 to the liquid/gas interface on the left side of the gauge plate 13, as viewed in FIG. 2A, the liquid level reading is at approximately 80%, as represented by the handle 11 and pointer 12 in phantom line. Accordingly, a large variance in actual liquid level can occur simply by incorrect installation of the gauge plate 13. Under these circumstances, it may be difficult for an operator to determine with any acceptable degree of accuracy the actual level of liquid within the tank. This is of course problematic when the bobtail or other transport vehicle (schematically represented by numeral 15 in FIG. 2B) with wheels 16 and tank 2 are located on a level surface 17.
This problem is exacerbated when the vehicle 15 is located on the side of a hill or other sloped surface 18 with angle “B” as shown in FIG. 3B. In this instance, even if the gauge plate 13 is properly installed as in FIG. 3A, the liquid level reading between the right and left sides of the tank 2 can still be offset by the angle “B”, since the upper surface 19 of the liquid 14B will always be perpendicular to the direction of gravity, i.e. the upper surface 19 will always be horizontal. Accordingly, when the tip 9 of the tube 7 is located at the gas/liquid interface on the right side of the gauge plate 13 in FIG. 3A, the pointer 12 indicates the liquid level is at approximately 70% for example. However, when the handle 11 is rotated to move the tip 9 to the liquid/gas interface on the left side of the gauge plate 13, as viewed in FIG. 3A, the liquid level reading is at approximately 60%, as represented by the handle 11 and pointer 12 in phantom line. Accordingly, a large variance in the manually determined liquid level can occur simply by the vehicle being located on a sloped surface and/or the gauge plate being installed at an incorrect angle or orientation.
In addition to the above, roto-gauges can become damaged through vibration and shock due to the vehicle itself as well as travel over rough roads and highways, thereby causing further inaccuracies in liquid level determination. Errors in liquid level reading can also occur when too much or too little liquid is located within the tank.
It would therefore be advantageous to provide an improved liquid level gauge that overcomes one or more disadvantages of prior art devices.